1. Field of the Invention
The present invention relates generally to a fluid filled vibration-damping device designed to produce vibration-damping action based on the flow behavior of a non-compressible fluid sealed within it, and more particularly to a fluid filled type vibration-damping device suitable for use as an automotive engine mount or the like, for example.
2. Description of the Related Art
Fluid filled vibration-damping devices designed to produce vibration-damping action based on resonance or other flow behavior of a non-compressible fluid scaled within are known in the art, as vibration-damping devices intended for installation between components that make up a vibration transmission system. The structure of such vibration-damping devices is typically composed of a first mounting member and second mounting member connected by a main rubber elastic body, and having formed therein a pressure-receiving chamber a portion of whose wall is constituted by the rubber elastic body, and an equilibrium chamber a portion of whose wall is constituted by a flexible film. A non-compressible fluid is sealed within the pressure-receiving chamber and the equilibrium chamber, and there is furnished an orifice passage through which the pressure-receiving chamber and equilibrium chamber communicate with each other.
In a fluid filled vibration-damping device of this kind, particularly effective vibration-damping effect can be obtained in the tuning frequency band of the orifice passage. Consequently, application of such devices in automotive engine mounts and the like, which must have excellent vibration-damping effect against certain specific frequency bands such as idling vibration or engine shake, has been study.
Research conducted by the inventors has revealed that fluid filled vibration-damping devices of conventional construction experience sometimes a problem of noise and vibration being emitted by the vibration-damping device when a large vibrational load is input across the first mounting member and the second mounting member. Specifically, in an automobile employing fluid filled vibration-damping devices as engine mounts, noise and vibration may be produced to an extent noticeable to passengers when driving on ridged pavement, or over a speed bump or the like.
The occurrence of such noise and vibration is attributed to a phenomenon whereby, during input of abrupt vibrational load, fluid flow through the orifice passage between the pressure-receiving chamber and the equilibrium chamber cannot keep up, resulting in a transitory but very high level of negative pressure forming within the pressure-receiving chamber, whereupon bubbles (known as cavitation) form due to release and vaporization of dissolved gases from the sealed filled. The bubbles grow within the pressure-receiving chamber and then burst, producing a large shock at the time. It is thought that this large shook is transformed into water hammer pressure, which is transmitted to the first mounting member and the second mounting member, and then transmitted to the automobile body and so on, producing the problem of noise and vibration discussed above.
In order to address such problems, there has been proposed, for example in U.S. Pat. No. 4,697,793, a structure wherein a partition rubber film is disposed partitioning the pressure-receiving chamber and the equilibrium chamber, with a slit being formed in the partition rubber film. With this structure, in the event that a large pressure differential arises between the pressure-receiving chamber and the equilibrium chamber, the partition rubber film experiences a high level of elastic deformation, causing the slit to open up. This arrangement makes it possible to eliminate the pressure differential between the pressure-receiving chamber and the equilibrium chamber.
However, a problem with the structure proposed in U.S. Pat. No. 4,697,793 is that, since the slit of the partition rubber film will open up not just in the case that negative pressure has arisen in the pressure-receiving chamber, but also in the case that positive pressure has arisen. This becomes difficult to ensure adequate relative pressure fluctuations between the pressure-receiving chamber and the equilibrium chamber when vibration is input. As a result, it becomes difficult to ensure sufficient flow of fluid through the orifice passage, creating the problem that the desired vibration-damping action by the orifice passage may not be adequately achieved.
In view of the above problem, the applicant has proposed in JP-A-2003-148548, a structure employing valve means consisting of a lip-shaped rubber elastic body, whereby when a high level of negative pressure in excess of a set level of negative pressure should arise in the pressure-receiving chamber, the orifice passage will be short-circuited to the pressure-receiving chamber by means of a short-circuit passage formed in the partition member. With the structure according to this prior filing, the valve means functions as a one-way valve, so that pressure escape will be prevented even if high level of positive pressure should arise in the pressure-receiving chamber. Consequently, it becomes possible to ensure sufficient flow of fluid through the orifice passage, while avoiding the occurrence of excessive negative pressure in the pressure-receiving chamber, and suppressing the occurrence of noise and vibration thought to be caused by cavitation.
However, further research conducted by the inventors has revealed that the fluid filled vibration-damping device disclosed in JP-A-2003-148548 is not fully satisfactory in some instances. Specifically, since the valve means shown in JP-A-2003-148548 is constituted by a lip-shaped rubber piece, depending on the thickness dimension, size, rubber material or other settings thereof, there is a risk of deformation, deterioration, or breakage with repeated opening and closing operation, making it difficult to ensure sufficient endurance in some cases. Also, depending on the thickness dimension, size, rubber material or other settings of the rubber piece making up the valve means, there is a risk that in the event that a very high level of positive pressure should arise in the pressure-receiving chamber, the valve means will experience elastic deformation so as to become pushed towards the short-circuit passage side, and will no longer return, so that the short-circuit passage is left in a state of being constantly open.
To address this problem of the valve means disclosed in JP-A-2003-148548, it was contemplated to constitute the valve means from a rubber piece having ample thickness. However, increasing the thickness of the rubber piece making up the valve means results in a high spring constant and difficulty in deforming. This in turn makes it difficult to effectively and rapidly eliminate negative pressure arising in the pressure-receiving chamber, making the design difficult to adopt in actual practice.